Hi all,
We have a 2018 Mercury Optimax Pro XS 225hp (s/n 2B534312 model 1225P83BD) that when turning the ignition switch causes the 15 amp "Power-on Circuit" Fuse to (per the "Operation Maintenance and Installation Manual") blow. We had batteries tested, both bad, and have been replaced.

The back story: spent the summer/early fall replacing the fuel tank on our Mako. so the whole console had to come off, obviously needing to disconnect everything. Also took the opportunity to rewire electronics and a new switch panel. So yes, lots of electrical changes. But have checked ENGINE wires both under console and on block, all look new and in good shape. All were plugs and double checked in right place.

All electronics/lights were and are working fine. On the day we were to launch, no response from the engine. Did some research, found the blown fuse. Replaced.
Tested again (and not happy with myself afterwards for doing this) turned over the engine with no gas. (I am praying one inadvertent turnover would not hurt the fuel pumps.)
So we filled the boat with gas, launched, primed the bulb till hard, engine started right up.
Went for a ride - 5 minutes to open water, then on a plane for about 1-2 minutes at 22knots, and the engine stops dead. Fuse blown again. Replace, blows immediately.
We get a tow in (thankfully only couple hundred yards off shore). This is when we had batteries tested and replaced (read about possible cause of an increase in amperage for low voltage).
Now the fuse blows every time. It is immediate and sparks.

In trying to isolate, i have done following:
Disconnected SmartCraft gauges. Fuse blows.
Disconnected the new electronics switch panel off the batteries. Fuse still blows.
Disconnected high pressure fuel pump. Fuse blows. (so i guess pump is ok.)
Also have a service manual on order, hopefully here this week so have for next weekend.

Have read many posts about check wires, check wires, check wires, which we’ve done, as said above. All on block look in great shape, terminals not even corroded and no worn insulation. Could it be possible we put a screw through the conduit and into a wire? Possibly, but I don't think so.
Starting this thread to hopefully generate ideas where to look / next steps.

Thanks,
Mako538